I came across a cryptic reference in a 19th century volume of translations of liturgies to an East Syriac "Divine Liturgy of St. Thomas" used in the Indian church before the conversion to the West Syriac Rite, or perhaps by the Malabar Catholics, similiar to but different from the Liturgy of SS. Addai and Mari.

Now this might well, indeed very well probably, will emege as Addai and Mari with an institution narrative, but I thought it might be interesting to mention this potentially apocryphal (that is to say, non extant) service in light of Mor's view of an Orthodox East Syriac church in India.

There is one other sort of non-existant liturgy that 19th century liturgiologists thought existed, the "Liturgy of St. Clement," which if memory serves, is in fact that from the Apostolic Constitutions.